Various applications require filtering of multiple combined spectral components in signals by selecting one or more of the spectral components while rejecting the other components. One example is band pass filtering where a selected one or more spectral components within a spectral pass band are selected to transmit and spectral components outside the spectral band are rejected. A filter may be tunable, e.g., under a control of a tuning control signal, to change the frequency range of the filtered signal.
Optical band pass filters are known where optical spectral components within a spectral window transmit through the filter while other spectral components outside the spectral window are rejected. It is known to construct optical band pass filters using optical resonators, which are small electro-optical devices, having diameters on the order of millimeters, formed of curved optical waveguides, for example, a cylinder, a sphere, or a toroid within which light is internally reflected at the inner surface of the optical resonator. Some optical resonators can support resonator modes of light called whispering gallery modes, and thus, are often referred to as whispering gallery mode resonators. Whispering gallery modes occur when light having an evanescent wave component travels via internal reflection around the periphery of the optical resonator. The whispering gallery modes of optical resonators reside close to the surface of the optical resonator, and undergo total internal reflection. The evanescent wave component extends beyond the optical resonator's outer surface and may be coupled into an adjacent optical coupler as long as the optical coupler is located within the extent of the evanescent wave, typically on the order of the light's wavelength.
Many optical resonators which propagate whispering gallery modes of light have extremely low transmission losses, and as a result, have a very high quality factor Q. High Q optical resonators are desirable because the higher the Q, the longer the amount of time the internally reflected light will remain within the optical resonator.
Optical domain filters are able to filter any desired signal including RF, microwave, millimeter, Gigahertz or Terahertz frequency that is modulated as a sideband on an optical carrier. The use of whispering gallery mode resonator technology allows for designing optical domain filters with features of small size and weight, suitable for ground as well as spacecraft applications.
RF filtering using whispering gallery mode resonator technology requires wavelength locking between the optical carrier source and the optical resonators used to construct the filter. It is possible to achieve wavelength locking of semiconductor lasers to whispering gallery mode resonators with a zero spectral offset, but such an arrangement is not useful because the RF sideband spectrum is rejected along with the laser spectrum by the whispering gallery mode resonator filter. Some known wavelength locking implementations directly sample the carrier signal to create a reference signal prior to the addition of a modulated signal and later combine the sampled carrier signal with a filtered spectral component. However, such implementations require complicated time delay elements to compensate for group delay of the filtered spectral components. Further, such implementations are only useful for relatively short optical paths, and are unable to effectively maximize the carrier signal power for peak detection and locking. It is therefore desirable to develop a system and method for wavelength locking between the optical source and the optical resonators used to construct a band pass filter that fixes the optical source at an offset from the center of the band pass filter's passband, so that suppression of the optical source spectrum does not result in suppression of the RF sideband carrying the information of interest. It is further desirable to perform the filtering function and the wavelength locking function on a single input signal to eliminate time delay elements.
Additionally, some complex optical links apply multiple modulations to the optical source spectrum to accomplish frequency translation of the RF signal, as for example with a local oscillator (“LO”) signal modulating the optical signal. The complex optical links having multiple modulations present a complicated mix of RF and LO sidebands to the optical input of the filter, creating background interference noise that interferes with the ability to lock to, or even recognize, the presence of the optical carrier among the various sidebands. Therefore, it is desirable to develop a system and method for wavelength locking between the optical source and the optical resonators used to construct a band pass filter that isolates the optical carrier from multiple sidebands so that it can be used to lock the filter to the carrier, while at the same time creating an offset between the laser and the filter so that an RF signal of interest passes through the passband of the filter.